Various types of semiconductor memory devices are generally used. One type is a solid-state drive (SSD). The SSD has a plurality of NAND flash memories. NAND flash memories are nonvolatile memories, each capable of storing a great amount of data at high density. They are memory devices expected to be used in place of the hard disk drive.
Data can indeed be read from each NAND flash memory provided in the SSD. In some cases, however, data must be written into all NAND flash memories. This is why data is read from the SSD during a period, and is written into the SSD during another period not overlapping the first-mentioned period.
Since data cannot be read and written from and into the SSD in any overlapping periods, more time is spent to write and read data than otherwise. A demand is therefore made for a technique that can write and read at high efficiency.